The first is the story of the book. Manchester wrote two volumes of his Churchill triptych and had made strides toward the third when he suffered two strokes. It became evident that he could not continue a work that, more than his account of the assassination of John F. Kennedy (“Death of a President,’’ 1967), his history of modern America (“The Glory and the Dream,’’1974) or his biography of Douglas MacArthur (“American Caesar,’’ 1978), would be his highest achievement. He turned to Reid, a relatively unknown Palm Beach Post feature writer, who picked up the baton and, it must be said, finished the race with agility, grace, and skill.

The second is the book’s story. Churchill’s war years (followed by his coda as a Conservative prime minister in the early 1950s) constitute the greatest tale of the 20th century, one of determination, motivation, and adaptation. Churchill’s life is the story of how the indomitable will of one man, endowed with a lyrical mind and booming voice, could stand athwart history and all the probabilities of European geopolitics in the 1940s and say yes — to freedom, to military victory, to literary mastery.

“Defender of the Realm, 1940-1965’’ is a long book, and in truth a deft editor could have cut out half, very likely more, with ease. Let us applaud that none did. Churchill’s passage through World War II is the wheat of this vast field, but even the chaff is history itself, beautifully rendered. Every word Churchill spoke meant something, and so it is with this book, even at more than 1,100 pages. At the conclusion of this volume, as at the conclusion of Churchill’s life, the discerning may wish it were longer.

For this is a book that is brilliant and beautiful, evocative and enervating. The description of Paris in the spring of 1940; the account of the household habits of Churchill; the “atrophy of spirit’’ of the Allies, especially the French, weary of war even before it started, then wary of it once underway — all are sketched with surpassing skill, almost pointillist precision. No Churchill biography approaches it in sweep and accessibility — not his own accounts, perhaps not Martin Gilbert’s magisterial multivolume work, nor any of the multitude of books on Churchill’s leadership or relationships.

Churchill was a man of details, and Manchester a master of them, so these pages are generously seeded with telling detail such as the fact that Churchill worried about zoo animals subjected to Hitler’s bombardment and that Parisian firms exported silk that would reappear in France in the parachutes of invading German paratroopers.

And just as Churchill was a man of national sentiment, so Manchester (and Reid after his death) is a keen master of national character and identity, and so we learn, too, that as war approached France had no war plan, only a plan to avoid war or to repulse attack. (Here is how the France of that period is characterized: “France had no war aims. Everything desirable, as they saw it, was already French.’’) This, along with the Nazi hunger for land and the British grit for survival, are the central ideas of the European conflict.

Churchill was the product of a time, a class, and a country that he sought, vainly and with vanity, to freeze in time. “Upper-class Englishmen who had come of age then, when the empire stood at flood tide, possessed a certitude, an indominable faith in England, confidence in their own judgment, and an indubitable conviction that they understood the world and were its masters,’’ the authors write.

Like the brandy he preferred, Churchill was bottled in the 19th century, thrust incongruously but indispensably into the 20th, stubbornly referring to Constantinople when everyone else spoke of Istanbul, to Persia rather than Iran, and to Peiping rather than Peking, to say nothing of Beijing. Despite having a valet warm his brandy snifter and having almost no firsthand knowledge of the life of the ordinary Englishman, he preserved the English way of life. “He did not live in the past,’’ the authors write. “The past lived on in him.’’